When you need to check if an email address is valid or verify if a mailbox exists. You will probably want to use a free online email verification tool.
You need to enter an email address in a web form and let the service check it. Email verification service will make different tests to guarantee that you can deliver your message to this address. It will check for typos and errors. It will then connect to an email server and check that a mailbox exists.
The service will also tell if it is a one-time use mailbox. Or if the mailbox is hosted on free email service. The results help to make a better choice of keeping or removing the address from a mailing list.
If you send mass emails and need or want to ping a single mailbox, you can verify it online for free.
You can also use bulk email verification for mass email address verification. Or you may want to use an API and apply email checks to data entered on your website.